The present invention relates to a nonvolatile magnetic memory device and a method of writing data into a tunnel magnetoresistance device in a nonvolatile magnetic memory device. More specifically, the present invention relates to a nonvolatile magnetic memory device called a TMR (Tunnel Magneto resistance) type MRAM (Magnetic Random Access Memory) and a method of writing data into a tunnel magnetoresistance device in such a nonvolatile magnetic memory device.
With great diffusion of information communication machines, particularly, personal small machines such as personal digital assistances, various semiconductor devices such as a memory, a logic and so on, constituting such machines are being demanded to cope with higher performances such as a higher degree of integration, faster operation capability and lower power consumption. Particularly, a nonvolatile memory is considered indispensable in the ubiquitous era. Even if the depletion of a power supply or some troubles occur or a server is disconnected to a network due to some failure, important information can be stored or protected with a nonvolatile memory. Further, recently available personal digital assistances are designed such that the power consumption is reduced to a lowest level possible by maintaining non-operating circuit blocks in a standby state, and the waste of power consumption and a memory can be avoided if a nonvolatile memory capable of working as a fast-speed work memory and a mass-storage memory can be realized. Further, if a fast-speed and mass-storage nonvolatile memory can be realized, the “instant-on” function of booting in the instance of turning on power can be made possible.
The nonvolatile memory includes a flash memory using a semiconductor material and a ferroelectric nonvolatile semiconductor memory (FERAM, Ferroelectric Random Access Memory) using a ferroelectric material. However, the flash memory has a defect that the writing speed is slow since it is in the order of microseconds. On the other hand, in FERAM, the number of times of re-writability thereof is 1012 to 1014, and the number cannot be said to be sufficient for replacing SRAM or DRAM with FERAM, and there is pointed out another problem that the micro-fabrication of a ferroelectric layer is difficult.
As a nonvolatile memory free of the above defects, a nonvolatile magnetic memory device called MRAM (Magnetic Random Access Memory) is in the limelight. The MRAM at an early development stage was based on a spin valve using a GMR (Giant MagnetoResistance) effect. Since, however, the memory cell resistance against a load is as low as 10 to 100 ohms, the power consumption per bit on readout is large, and the defect is that it is difficult to attain the capacity of mass storage.
While the MRAM using a TMR (Tunnel MagnetoResistance) effect only had a resistance change ratio of 1–2% at room temperature at an early development stage, it has come to be possible to obtain a resistance change ratio close to 20% in recent years, so that the MRAM using the TMR effect is highlighted. The TMR-type MRAM has a simple structure and enables easy scaling, and recording is made by the rotation of a magnetic moment, so that the number of times of possible re-writing is great. Further, it is expected that the TMR-type MRAM is very rapid with regard to an access time period, and it is already said that the TMR-type MRAM is capable of an operation at 100 MHz.
FIG. 5 shows a schematic partial cross-sectional view of a TMR-type MRAM (to be simply referred to as “MRAM” hereinafter). The MRAM comprises a transistor for selection TR constituted of a MOS-type FET and a tunnel magnetoresistance device TMJ.
The tunnel magnetoresistance device TMJ has a stacking structure constituted of a first ferromagnetic layer 31, a tunnel barrier 34 and a second ferromagnetic layer 35. More specifically, the first ferromagnetic layer 31 has a two-layer structure, for example, of an anti-ferromagnetic layer 32 positioned below and a ferromagnetic layer (called a reference layer or a pinned magnetic layer 33 as well) positioned above and has an intense unidirectional magnetic anisotropy due to an exchange interaction working between these two layers. The second ferromagnetic layer 35 of which the magnetization direction rotates relatively easily is also called a free layer or memory layer. The second ferromagnetic layer will be called a memory layer 35 hereinafter. The tunnel barrier 34 works to cut a magnetic coupling between the memory layer 35 and the pinned magnetic layer 33, and a tunnel current flows in the tunnel barrier 34. A bit line BL for connecting the MRAMs is formed on a third insulating interlayer 26. A top coating film 36 formed between the bit line BL and the memory layer 35 works to prevent mutual diffusion of atoms constituting the bit line BL and atoms constituting the memory layer 35, to reduce a contact resistance and to prevent the oxidation of the memory layer 35. In Figure, reference numeral 37 indicates a lead-out electrode connected to the lower surface of the anti-ferromagnetic layer 32.
Further, a write-in word line RWL is arranged below the tunnel magnetoresistance device TMJ through a second insulating interlayer 24. Generally, the extending direction (first direction) of the write-in word line RWL and the extending direction (second direction) of the bit line BL cross each other at right angles.
The transistor for selection TR is formed in that portion of a semiconductor substrate 10 which portion is surrounded by a device isolation region 11, and the transistor for selection TR is covered with a first insulating interlayer 21. One source/drain region 14B is connected to the lead-out electrode 37 of the tunnel magnetoresistance device TMJ through a connecting hole 22 constituted of a tungsten plug, a landing pad 23 and a connecting hole 25 constituted of a tungsten plug. The other source/drain region 14A is connected to a sense line 16 through a tungsten plug 15. In FIG. 5, reference numeral 12 indicates a gate electrode, and reference numeral 13 indicates a gate insulating film.
In an MRAM array, the MRAM is arranged in an intersecting point (overlap region) of the bit line BL and the write-in word line RWL.
When data is written into the above-constituted MRAM, a current IBL is passed through the bit line BL and a current IRWL is passed through the write-in word line RWL, to form a synthetic magnetic field, and the direction of magnetization of the second ferromagnetic layer (memory layer 35) is changed by means of the synthetic magnetic field, whereby “1” or “0” is recorded into the second ferromagnetic layer (memory layer 35).
Data is read out by bringing the transistor for selection TR into an ON-state, passing a current through the bit line BL and detecting a tunnel current change caused by a magnetoresistance effect with the sense line 16. When the magnetization direction of the memory layer 35 and the counterpart of the pinned magnetic layer 33 are the same, a low-resistance state results (this state represents, for example, “0”), and when the magnetization direction of the memory layer 35 and the counterpart of the pinned magnetic layer 33 are antiparallel, a high-resistance state results (this state represents, for example, “1”).
FIG. 38 shows an asteroid curve of the MRAM. A current is passed through the bit line BL and a current is passed through the write-in word line RWL, and as a result, a synthetic magnetic field is generated. Data is written into the tunnel magnetoresistance device TMJ constituting the MRAM on the basis of the synthetic magnetic field. A magnetic field (HEA) in the easy-magnetization axis direction of the memory layer 35 is formed due to a writing-in current flowing in the bit line BL, and a magnetic field (HHA) in the difficult-magnetization axis direction of the memory layer 35 is formed due to a current flowing in the write-in word line RWL. In some MRAM constitution, a magnetic field (HHA) in the difficult-magnetization axis direction of the memory layer 35 is formed due to a writing-in current flowing in the bit line BL, and a magnetic field (HEA) in the easy-magnetization axis direction of the memory layer 35 is formed due to a current flowing in the write-in word line RWL.
The asteroid curve shows an inversion threshold value of magnetization direction of the memory layer 35 due to the synthetic magnetic field (synthesis of magnetic field vectors of the magnetic field HA and the magnetic field HEA to be exerted on the memory layer 35). When a synthetic magnetic field corresponding to an outside (OUT1, OUT2) of the asteroid curve is generated, the magnetization direction of the memory layer 35 is inverted, so that data writing is performed. When a synthetic magnetic field corresponding to an inside (IN) of the asteroid curve is generated, the inversion of magnetization of the memory layer 35 does not take place. Further, onto the MRAM other than the MRAM which is positioned in the overlap region of the write-in word line RWL and the bit line BL in which the currents are flowing, a magnetic field is additionally exerted by the write-in word line RWL or bit line BL alone, and when such a magnetic field is greater than a unidirectional inversion magnetic field HK [in a region (OUT2) outside dotted lines in FIG. 38], the magnetization direction of the memory layer 35 constituting the MRAM other than the MRAM which is positioned in the overlap region is also inverted. Therefore, only when the synthetic magnetic field is outside the asteroid curve and is in a region (OUT1) inside the dotted lines in FIG. 38, selective writing into the selected MRAM is possible.
As described above, TMR-type MRAM has an advantage that a higher speed and a higher degree of integration are easily attained. Actually, however, a magnetic field generated during writing data into a certain MRAM may destroy data stored in MRAM adjacent to the above MRAM.
As shown in FIG. 39, there is assumed a state where three conductor lines (L1, L0, L2) have an infinite length, these conductor lines are placed apart side by side at a distance of d, a current of I0 (ampere) flows in the conductor line L0 and a current of −I0 (ampere) flows in each of the conductor lines L1 and L2. If distances from an arbitrary point P (X,Y) to the conductor lines L0, L1, L2 are r0, r1 and r2, the magnetic fields HX and HY in the directions of X axis and Y axis can be determined on the basis of the following expressions (3-1) and (3-2). Angles at which the straight lines connecting the arbitrary point (X,Y) and the conductor lines L0, L1, L2 form with the X axis are θ0, θ1 and θ2. In FIG. 39, further, the magnetic field generated by the conductor line L0 is represented by an arrow H0, the magnetic field generated by the conductor line L1 is represented by H1, and the magnetic field generated by the conductor line L2 is represented by an arrow H2.
                                                                        H                X                            =                            ⁢                                                                    [                                                                  I                        0                                            /                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      r                            0                                                                          )                                                              ]                                    ⁢                                      sin                    ⁡                                          (                                              θ                        0                                            )                                                                      +                                                                                                      ⁢                                                                    [                                                                  I                        1                                            /                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      r                            1                                                                          )                                                              ]                                    ⁢                                      sin                    ⁡                                          (                                              θ                        1                                            )                                                                      +                                                                                                      ⁢                                                [                                                            I                      2                                        /                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  r                          2                                                                    )                                                        ]                                ⁢                                  sin                  ⁡                                      (                                          θ                      2                                        )                                                                                                          (                  3          ⁢                      -                    ⁢          1                )                                                                                    H                Y                            =                            ⁢                                                                    -                                          [                                                                        I                          0                                                /                                                  (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          r                              0                                                                                )                                                                    ]                                                        ⁢                                      cos                    ⁡                                          (                                              θ                        0                                            )                                                                      -                                                                                                      ⁢                                                                    [                                                                  I                        1                                            /                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      r                            1                                                                          )                                                              ]                                    ⁢                                      cos                    ⁡                                          (                                              θ                        1                                            )                                                                      -                                                                                                      ⁢                                                [                                                            I                      2                                        /                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  r                          2                                                                    )                                                        ]                                ⁢                                  cos                  ⁡                                      (                                          θ                      2                                        )                                                                                                          (                  3          ⁢                      -                    ⁢          2                )            wherein,r0=(X2+Y2)1/2r1={(X+d)2+Y2}1/2r2={(X−d)2+Y2}1/2
As shown in FIG. 5, a distance from the center of the bit line BL in the thickness direction to the center of the second ferromagnetic layer (memory layer) 35 is assumed to be “h”, and a distance from the center of the bit line in the width direction to the center of an adjacent bit line in the width direction is assumed to be “d”. And, it is assumed that β=(h/d).
The conductor lines L0, L1, L2 are assumed to be three bit lines arranged in parallel in the X axis direction, the conductor line L0 is assumed to pass the origin (0,0) of the Gauss' coordinate, and it is assumed that I1=0 (ampere). In this case, the value of H(x,h) at a point represented by a coordinate (X,h) is calculated according to the following expression (4).
                                                                        H                                  (                                      X                    ,                    h                                    )                                            =                            ⁢                                                [                                                            I                      0                                        /                                          (                                              2                        ⁢                        π                                            )                                                        ]                                ×                                  [                                      1                    /                                                                  (                                                                              X                            2                                                    +                                                      h                            2                                                                          )                                                                    1                        /                        2                                                                              ]                                ×                                  [                                      h                    /                                                                  (                                                                              X                            2                                                    +                                                      h                            2                                                                          )                                                                    1                        /                        2                                                                              ]                                                                                                        =                            ⁢                                                h                  ⁡                                      [                                                                  I                        0                                            /                                              (                                                  2                          ⁢                          π                                                )                                                              ]                                                  /                                  (                                                            X                      2                                        +                                          h                      2                                                        )                                                                                        (        4        )            
In the above expression, I0 is normalized such that when X=0, H(x,h)=1. In this case, the value of I0 is as follows.I0=2n·h  (5)
The expression (5) is substituted in the expression (4) to give the following expression (6). A normalized magnetic field H(X,h) is represented by HN(X,h).HN(X,h)=h2/(X2+h2)  (6)
When d·x is denoted by X, the expression (6) is substituted in the expression (7), using β=(h/d).
                                                                        H                                  N                  ⁡                                      (                                          X                      ,                      h                                        )                                                              =                            ⁢                                                                    (                                          β                      ·                      d                                        )                                    2                                /                                  {                                                                                    (                                                  d                          ·                          x                                                )                                            2                                        +                                                                  (                                                  β                          ·                          d                                                )                                            2                                                        }                                                                                                        =                            ⁢                              β                /                                  (                                                            x                      2                                        +                                          β                      2                                                        )                                                                                        (        7        )            
FIG. 40 shows HN(x,h) that are calculation results from the expression (7) when the value of β=(h/d) is 0.1, 0.5 and 1.0.
When the value of β=(h/d) is 0.1, that is, when the distance “d” from the center of the bit line in the width direction to the center of the adjacent bit line in the width direction is 10 times the distance “h” to the center of the second ferromagnetic layer 35 in the thickness direction, HN(x,h) in the case of x=±1.0, that is, in the second ferromagnetic layer 35 of the adjacent tunnel magnetoresistance device TMJ, is nearly 0, and there is no interference of the magnetic fields between adjacent tunnel magnetoresistance devices TMJ.
However, when the value of β=(h/d) is 0.5, that is, when the distance “d” from the center of the bit line in the width direction to the center of the adjacent bit line in the width direction is 2 times the distance “h” to the center of the second ferromagnetic layer 35 in the thickness direction, HN(x,h) in the case of x=±1.0, that is, in the second ferromagnetic layer 35 of the adjacent tunnel magnetoresistance device TMJ, is 0.2. Whether or not this value of HN(x,h) poses a problem depends upon fluctuation of the magnetic field (HEA) in the easy-magnetization axis direction of MRAM and fluctuation of the magnetic field (HHA) in the hard-magnetization axis direction of MRAM in the asteroid curve of MRAM shown in FIG. 38. However, the value has a magnitude that cannot be ignored.
Further, when the value of β=(h/d) is 1.0, that is, when the distance “d” from the center of the bit line in the width direction to the center of the adjacent bit line in the width direction is 1 time the distance “h” to the center of the second ferromagnetic layer 35 in the thickness direction, HN(x,h) in the case of x=±1.0, that is, in the second ferromagnetic layer 35 of the adjacent tunnel magnetoresistance device TMJ, comes to be as large as 0.5. When such a magnetic field is generated, it is expectedly difficult to write data stably into a predetermined MRAM even if the fluctuation of the magnetic field (HEA) in the easy-magnetization axis direction of MRAM and the fluctuation of the magnetic field (HHA) in the hard-magnetization axis direction of MRAM can be controlled to be small.
In “Present State and Future View of MRAM” by Yoshiaki SAITO, FIG. 6 shows that when X=0 μm, Hx≈10 Oe, when X=0.2 μm, Hx≈5 Oe, and when X=0.4 μm, Hx≈2 Oe. That is, the value in the case of β=h/d=1.0 in FIG. 40 is more or less correspondent to the data shown in FIG. 6 of “Present State and Future View of MRAM” by Yoshiaki SAITO.
Although it is undeniable that the values differ when boundary conditions, etc., are strictly taken into account, it can be affirmed that even the model depicted by the expression (7) gives a good approximation for the purpose of studying the distribution and magnitude of the magnetic field.
While TMR-type MRAM has an advantage that a higher speed and a higher degree of integration are easily attained as described above, a magnetic field generated during writing data into a certain MRAM may destroy data stored in MRAM adjacent to the above MRAM as discussed above.
For example, JP-A-2002-20388 discloses a means of overcoming the above problem. In the technique disclosed in the above Japanese laid-open patent publication, programming currents (IWL, IBL2) are passed through a word line (WL1) and a bit line (BL2) constituting a memory cell (I2). A compensatory current that provides a compensatory magnetic field to counteract a scattered magnetic field is passed through a word line (PWL) or a bit line (BL3,BL5) constituting at least one memory cell (I3,I5) adjacent to the memory cell (I2).
However, the above Japanese laid-open patent publication describes no specific method or means with regard to what value should be employed for the compensatory current to flow or how the value of the compensatory current to flow should be determined.
Further, the above Japanese laid-open patent publication is silent concerning any specific method of simultaneously writing data in many adjacent MRAMs.